The current method of making nitrides of uranium is an extremely complicated one which requires that the pure metal be first obtained by appropriate reduction steps. The metal is then converted to a fine powder by successive hydriding and dehydriding operations and the powder is finally nitrided under controlled temperature and pressure conditions. It is an object of this invention to provide a method for obtaining uranium nitrides which is more convenient than any known method, a further object being to provide an equally attractive method for converting said nitrides back to uranium metal.
The long-term operation of the present actinide-fueled reactors is seriously handicapped in that the fuel must be removed and repurified after relatively small amounts of fuel, e.g., about 0.1 to 2.0 percent, are burned. While this disadvantage can in part be minimized by over-sizing the reactor so as to provide for an unnecessarily large concentration of fuel, this is obviously an uneconomic expedient. In repurifying the fuel, wet-chemistry or pyrometallurgical methods have been utilized. Current wet-chemistry separations require long periods of radioactive cooling for the radioactivity to decay, followed by a series of complexing, chelation and ion exchange steps which generate considerable quantities of radioactive wastes. Disposal of the latter poses a serious problem.
It is recognized that pyrometallurgical purification methods should be used if at all possible since they are fast and do not require the use of long radioactive cooling periods. However, no practical pyrometallurgical method has been developed which is as effective as the wet-chemical methods. For example, the complicated schemes proposed for extracting various of the metal impurities with liquid metal or a molten salt result in the loss of a significant amount of the fuel material. It has also been proposed to convert the metals to fluorides and then to distill off the uranium present in the form of uranium hexafluoride. This method has the disadvantages that one is working with toxic and corrosive gases and that the method requires expensive processing equipment for its operation. It is, therefore, a further important object of this invention to provide a pyrometallurgical method for reprocessing and purifying any type of actinide fuel (whether metal, oxide, sulfide, carbide, silicide or nitride, for example) which is contaminated with fission products, said method being fast, safe of operation, inexpensive and highly efficient from a fuel recovery standpoint, while also providing a highly concentrated radioactive waste product.
A still further object of the invention is to provide a novel reactor system and process of operating the same which permits of the continuous and automatic in situ removal of fission products, including those of a gaseous nature, as the operation continues; which similarly permits of the addition of new fuel components (including fertile materials, if desired); and which in one embodiment thereof is inherently stable and safe of operation in the sense of being self-correcting as reaction temperatures rise or fall from equilibrium levels. Another object is to provide a method which permits uranium to be separated from plutonium when the latter is present in relatively small amounts.